1. Field of the Invention
This invention relates to a superconductor wire using an oxide superconductor, and to a method of manufacturing such a superconductor wire.
2. Background of the Invention
The method of manufacturing a superconductor wire by making use of an oxide superconductor can be classified mainly into the following two kinds of methods. A first method includes the steps of filling a polycrystalline metallic sheath with an oxide superconductor powder, and drawing or rolling the sheath, whereby producing a tape-like wire. According to this method, the superconductor powder charged in the metallic sheath is drawn together with the metallic sheath thereby to form a continuous and uniform precursor of superconductor tape. This precursor is suitably heat-treated during or after the working thereby subjecting the oxide superconductor powder to a reaction, thus obtaining a tape-like wire of oxide superconductor.
A typical example of the tape-like superconductor wire to be obtained by this method is the one wherein a Bi-based oxide superconductor (Bi2212, Bi2223) is joined and covered by making use of a polycrystalline silver or silver alloy sheath (ISTEC Journal, Vol. 8, No. 2, 1995, pp. 46-51). This wire is not preferable, since the crystal orientation degree of c-axis of the oxide superconductor is about 90%, and the crystal matching of a-axis and b-axis is 60% or less. Due to these poor properties, the critical current density J.sub.c of the Bi2223 type wire under the conditions of 77 K and zero tesla is at most 4 to 6.times.10.sup.4 A/cm.sup.2. However, because of its high manufacturing speed of 100 m/h, this manufacturing method of wire is attractive with respect to an industrial viewpoint.
A second method comprises the steps of forming an intermediate layer controlled in crystal orientation on the surface of a continuous metallic tape, and forming an oxide superconductor on the surface of the intermediate layer. In this method, the intermediate layer plays an important role in particular. Namely, this method is featured in that the intermediate layer functions not only to suppress the diffusion reaction between the metallic layer and the oxide superconductor layer being formed, but also to easily control the orientation of the oxide superconductor layer formed thereon since the intermediate layer is already controlled in crystal orientation. As a result, the integrity between the crystal grains can be improved, whereby making it possible to easily achieve such a high critical current density that the other conventional methods have failed to achieve.
A typical example of the tape-like superconductor wire to be obtained by this method is the one wherein a stabilized zirconia (YSZ) controlled in crystal orientation is formed on a Hastelloy tape by means of an ion beam-assisted deposition (IBAD) method so as to obtain the stabilized zirconia which is orientated in c-axis and aligned in a- and b-axis (in-plane orientation), and then a Y123 (YBa.sub.2 Cu.sub.3 O.sub.7-y)-based oxide superconductor thin film is formed on the stabilized zirconia by means of a laser ablation method. Since this tape-like wire is high in crystal matching degree of a- and b-axis, the critical current density J.sub.c thereof is as high as 0.5 to 1.0.times.10.sup.6 A/cm.sup.2 under the conditions of 77 K and zero tesla.
However, this method is defective in that the film-forming speed is as low as 0.001 to 0.01 m/h, thus raising many problems with respect to an industrial viewpoint in the manufacture of a continuous wire (Y. Iijima et al., Appl. Phys. Lett. Vol. 60 (1992) 769).
There is also known the RABiTS method which is similar to the aforementioned method (D. P. Norton et al., Science Vol. 274 (1996) 755). According to this method, a nickel tape which is textured-structured is formed at first by applying a rolling and a heat treatment to nickel metal, and then Ag or Pd is deposited on the surface of the textured-structured nickel tape by means of an electron beam deposition, or CeO.sub.2 or YSZ is deposited on the surface of the textured-structured nickel tape by means of a sputtering method, thereby forming an in-plane-orientated intermediate layer. Subsequently, a Y123-based thin film is formed on the intermediate layer by means of a laser ablation method. Since the tape-like wire to be obtained by this method is also high in crystal matching degree of a- and b-axis, the J.sub.c thereof is as high as 0.7.times.10.sup.6 A/cm.sup.2 under the conditions of 77 K and zero tesla.
However, this method requires the repetition of a complicated process for forming a thin film in order to obtain the intermediate layer, thus inviting many problems when it is applied to the manufacture of actual wire, such as a high manufacturing cost and a low manufacturing speed of the wire. Further, although it is possible with the aforementioned methods to obtain a superconductor film of high J.sub.c, the quality of the film may be deteriorated or a crack may be generated in the film as the thickness of the film is increased by making use of these methods, thus making it difficult to increase the I.sub.c.
There is also reported a method wherein a nickel or copper tape is subjected to an oxidation treatment so as to form an oxide layer as an intermediate layer on the surface of the tape, and then an oxide superconductor layer is formed on this intermediate layer (A. Ginsbach et al., Physica CI85-189 (1991) 2111). This method is designed to utilize an oxide film which has been formed on the surface of a metallic tape by merely oxidizing the metallic tape as an intermediate layer which is equivalent to the aforementioned YSZ or CeO.sub.2. Therefore, this method is suited for use in mass production and can be said to be a practical method.
However, the improvement in orientation of an oxide crystal is not taken into account in this method, so that the J.sub.c of the oxide superconductor layer formed on this oxide layer by means of a sputtering method is at most 1.times.10.sup.3 A/cm.sup.2, which is lower by about three digits (thousandth) as compared with the aforementioned high orientation tape wire.
There is also proposed a method wherein Ag is subjected to a rolling and a heat treatment so as to form a textured-structure therein, and then an oxide superconductor layer is directly formed on the surface of the textured-structured Ag (ISTEC Journal Vol. 8, No. 2, 1995, pp. 44-46). According to this method, the {100} plane of Ag which is in-plane-orientated can be obtained ultimately. It is reported that since the lattice constant of Ag is about 4.09 angstroms which is close to the lattice constant of the c-axis of the oxide superconductor crystal, it is possible to obtain an epitaxial film by growing an oxide superconductor by means of vapor phase growth on this crystal surface.
The tape wire to be obtained by making use of this method shows a critical current density J.sub.c of 1 to 10.times.10.sup.4 A/cm.sup.2 under the conditions of 77 K and zero tesla if a thallium-based oxide superconductor is employed. However, since it is inherently difficult to transform Ag into a textured structure, any further improvement of the orientation is not feasible. Further, since the melting point of Ag itself is about 960.degree. C. in an air atmosphere, which is close to the melting point of the oxide superconductor, Ag may be melted in a film-forming method such as a liquid phase epitaxial method employing a high temperature of nearly 1,000.degree. C. (ISTEC Journal Vol. 8, No. 2, 1995, pp. 18-22).
The superconductor wire employing a Bi2223 type superconductive material according to the aforementioned prior art is accompanied with a problem that J.sub.c under a condition of 77 K is low, so that the application thereof to a superconductor device to be actuated at 77 K is limited. Further, the IBAD method for manufacturing a Y123 type tape-like wire is also accompanied with problems that a process to be performed in vacuum is involved in the manufacture of an YSZ intermediate layer having a uniform crystal orientation, that since the method is based on a vapor phase method, the film-forming speed is low, and that the crystallinity is deteriorated as the film thickness is increased thereby to lower the value of J.sub.c. Therefore, it is very difficult to manufacture a continuous tape-like wire for practical use by making use of these conventional techniques.
On the other hand, the conventional method of oxidizing the surface of nickel or copper and employing a resultant oxide layer formed on the metal as an intermediate layer is defective in that it is difficult to obtain a desired orientation in the Y123 type crystal, so that the J.sub.c cannot be improved. As for the utilizing the textured structure of silver, it is essentially difficult to obtain a textured structure of silver which is excellent in orientation, so that the J.sub.c cannot be increased. Moreover, since the melting point of silver is relatively low, a film-forming method which requires a high temperature cannot be applied to this case. Furthermore, since Ag can be easily softened, a sufficient strength for use as a wire cannot be obtained, thus being limited in end-use.